Such transmission is performed by means of an optical emitter connected to an optical receiver by means of the fiber. The emitter uses an optical carrier wave and it modulates the power thereof as a function of the information to be transmitted. As a general rule, modulation consists in varying the power of the carrier wave between two levels: a low level corresponding to the wave being extinguished, and a high level corresponding to the maximum optical power of a laser oscillator. Conventionally, the low and high levels represent binary values "0" and "1", respectively. The variations in the level of the wave are triggered at instants that are imposed by a clock signal which thus defines successive time cells allocated to the data to be transmitted.
In general, the maximum transmission distance is limited by the ability of receivers to detect these two power levels without error after the modulated wave has propagated over the optical link. To increase this distance, attempts are generally made to increase the ratio between the mean high optical power level and the mean low optical power level, with this being referred to as the "extinction ratio" and constituting one of the characteristics of the modulation.
Also, for a given distance and a given extinction ratio, the data rate is limited by the chromatic dispersion generated within the fibers. This dispersion which results from the effective refractive index of the fiber depending on the wavelength of the wave it is conveying has the consequence of emitted pulses increasing in width as they propagate along the fiber.
To limit the consequences of that phenomenon, proposals have been made to reduce the spectrum band width of the signal to be transmitted by means of appropriate coding. In particular, proposals have been made to use "duobinary" code which is well known in the field of electrical transmission. This code has the property of halving the spectrum width of the signal. In this code, a three-level signal is used with the levels being symbolized respectively as 0, +, and -. Binary digit "0" is coded by level 0, while binary digit "1" is coded either by level + or by level -, using the coding rule whereby the levels coding two successive blocks of "1" respectively surrounding an even number or an odd number of successive "0" are respectively identical or different.
The use of duobinary code for optical transmission was mentioned in an article entitled "10 Gbit/s unrepeatered three-level optical transmission over 100 km of standard fiber" by X. Gu et al., Electronics Letters, Dec. 9, 1993, Vol. 29, No. 25. According to that article, the three levels 0, +, and - correspond respectively to three levels of optical power.
French patent application No. 94 047 32, published under the No. FR-A-2 719 175 also describes duobinary coding applied to an optical system. In that document, binary digit "0" always corresponds to a low level of optical power, while the symbols + and - both correspond to the same high level of optical power, but differ by the phase of the optical carrier being shifted through 180.degree..
The use of such duobinary code with phase inversion is also mentioned in the article "Optical duobinary transmission system with no receiver sensitivity degradation" by K. Yonenaga et al., Electronics Letters, Feb. 16, 1995, Vol. 31, No. 4.
Although those articles concerning experimentation with that code report an improvement over conventional non-return to zero (NRZ) code, such improvement is not always observed. Thus, when conditions for implementing the code are close to ideal, in particular when using the highest possible extinction ratio, it ought to be observed that improvement is at a maximum. Paradoxically, simulations and tests have given results contrary to those which were expected.
If the physical effects of duobinary code are examined in detail in the context of an optical system, it can be observed that a reduction in the spectrum width of the signal is indeed obtained. However, the code has no influence on the spectrum of each pulse considered in isolation, whereas that is the determining factor concerning the effects of chromatic dispersion.
The positive results mentioned in the various articles are difficult to explain. Although some of the experimental parameters are verifiable (length and quality of the fiber, data rate), other parameters cannot be monitored with precision: characteristics of the optical components and real operation of the electronic monitoring circuits.
After simulations and testing in which the experimental parameters were varied, it turns out that an improvement is obtained providing a phase shift of the carrier wave occurs within each "0" preceding or following each block of "1s" or each isolated "1". The absolute value of the phase shift may be about 180.degree.. Also, it is important to avoid the low level power encoding the "0s" being as small as possible, i.e. to avoid the extinction ratio being as large as possible. In practice, an optimum value for the extinction ratio is a complex function of other experimental parameters. By way of example, it can be selected to be no greater than 20.
It is therefore appropriate to make an emitter device that is capable of applying a phase shift of the order of 180.degree. to the carrier wave within each cell that corresponds to a logic "0", and which precedes or follows any consecutive block of cells corresponding to logic "1" or indeed any isolated cell corresponding to a logic "1".
For this purpose, it is possible to use a laser oscillator coupled to an optical power modulator which is itself coupled to a phase modulator. By applying appropriate electrical control signals to the modulators, the optical power modulator delivers a wave to the phase modulator at an amplitude that is variable and that is carried by the wavelength of the laser, and the phase modulator outputs a wave that is modulated both in power and in phase.
In a variant, it is also possible to use a laser oscillator that is optically coupled to a power modulator. The assembly can be constituted simply by a known type of integrated modulator laser. Unlike the above embodiment, phase modulation is now obtained by acting on the laser injection current. This embodiment makes use of the property of lasers whereby they oscillate at a frequency that varies as a function of injection current. In an optimized embodiment, the laser is designed so that a small variation in current gives rise to sufficient variation in frequency without causing the power of the emitted wave to be subject to significant fluctuation.
Nevertheless, both of those solutions suffer from the drawback of requiring electronic control that is complex and expensive.
In order to simplify control, it is possible to make use of the fact that the phase shift can be performed every time a cell contains a logic "0" and to make use of an interferometer modulator of the "Mach-Zehnder" type. Such a modulator comprises an interferometer structure constituted by an inlet light guide which is subdivided into two branches that are recombined to form an outlet guide. Electrodes are provided for applying respective electric fields across the two branches. When the inlet light guide receives a carrier wave at constant power, two partial waves propagate along the two branches, and then interfere at the outlet. The outlet guide then provides a wave whose power and phase depend on the values of the electric voltages applied to the electrodes. To create a wave that is modulated in power and in phase, a voltage is applied to at least one of the electrodes, which voltage is amplitude modulated in a manner that corresponds to the binary signal to be emitted.
Since the phase changes need to take place at instants when the power of the emitted wave has a minimum value, it is appropriate to bias the electrodes so that in the absence of modulation, the DC components of the applied electrical voltages are such that the interference between the two partial waves is as destructive as possible. If the modulator has two identical branches, this condition implies that the DC components should be different.
The modulator can be made on a substrate of lithium niobate LiNbO.sub.3. Nevertheless, modulators on LiNbO.sub.3 are not suitable for integration, they are expensive, and they age poorly. It is possible to envisage using an interferometer modulator having the same configuration but made on a substrate of III-V elements, such as indium phosphide (InP). Nevertheless, that transposition is unsatisfactory since, unlike lithium niobate, attenuation in the guides due to the non-linear electro-optical effects in the III-V elements is highly dependent on the applied voltage.